From their website:
This summer, Fantasy Flight Games is excited to expand your Lannister forces for Battles of Westeros with Wardens of the West. The coffers of Casterly Rock afford better armor and superior weaponry for its loyal troops. Now is your chance to bolster your Lannister ranks.
Wardens of the West is a Lannister reinforcement set that introduces three new Lannister commanders – Tyrion Lannister, Sandor Clegane, and Daven Lannister – each with their own special abilities and leadership cards. Just like the commanders in the core set, these new commanders have specific strengths (and weaknesses). For example, Sandor “the Hound” Clegane uses the new fearsome keyword, refusing to let his enemies disengage and forcing them to retreat two hexes for each morale result rolled in combat.